Polyoma virus functions expressed by the early region of the genome will be studied. Altered viruses lacking restriction endonuclease sites in the early portion of the genome have been isolated, and are being characterized with respect to the type(s) of mutational alterations, and to the effects of these alterations upon the biological properties of these viruses. The ability of these viruses to a) replicate DNA, b) stimulate host cell DNA synthesis, c) induce cell transformation (abortive and stable) and d) produce T-antigen will be characterized.